An electronic component is a component taking the form of a package on the external face of which pins for connecting to an external electrical circuit are provided. Such a package comprises one or more electronic chips and electrical connections for connecting these one or more electronic chips to the connecting pins. The one or more electronic chips and the electrical connections are encapsulated in a given encapsulating material intended to protect them from the external environment. Typically, the encapsulating material is an electrically insulating polymer. Commonly, this encapsulating material also forms the external walls of the package.
Each electronic chip is a piece of a semiconductor substrate frequently called a wafer. These electronic chips are fabricated using wafer-scale microelectronic fabrication processes. These processes especially consist in depositing and etching layers in succession, one on top of the other, in order to “functionalize” the semiconductor substrate, i.e. to provide the electronic chip with the desired function.
Many of these chips act as controllable switches. This is because this is one of the basic functions of electronics. These electronic chips switch between an on state and an off state in response to a control signal. For this purpose, electronic chips generally comprise a semiconductor substrate essentially lying in a plane and having an active zone formed by at least one p-doped region and at least one n-doped region, which form one or more p-n junctions through which most of the useful current flows when this electronic chip is in an on state. The active zone is also called the drift region.
In the on state, the active zone has a low resistance to the useful current flow, which flows between at least two pins for connecting the electronic chip. In contrast, in the off or blocked state, the active zone has a much higher resistance to the useful current flow between the same connecting pins. Typically, the ratio U/J is higher than 1×106 Ω/cm2 and preferably higher than 1×108 Ω/cm2 in the off state, U being the voltage across the connecting pins and J being the current density flowing through the active zone. Ideally, the flow of useful current is completely stopped in the off state. In the on state the ratio U/J is lower than 1 Ω/cm2 and preferably lower than 0.01 Ω/cm2.
Switching between the on state and the off state can be controlled by a control signal delivered to a control electrode provided for this purpose. This control electrode is, for example, called a “gate” when the electronic chip is a field-effect transistor such as an IGBT (insulated gate bipolar transistor) or a MOS (metal oxide semiconductor) transistor, or a “base” in the case of a bipolar transistor, or even a “trigger” in the case of a thyristor. Switching between these two states may also only depend on the voltage applied across the connecting pins, as is the case for a diode. In the latter case, the control signal that makes the diode switch between the two on and off states is the voltage applied between its cathode and its anode.
Electronic chips are heated by Joule losses, which adversely affect their performance or limit their operating range, their current rating for example. The higher the product of the current and the voltage across the connecting pins, the greater the Joule heating. If no precautions are taken, Joule heating can destroy the electronic component, because of its temperature limits. Thus, it is almost essential to cool power electronic components, i.e. components capable of passing a useful current the density of which is higher than 1 A/cm2, and preferably higher than 100 A/cm2, for, for example, ten minutes or even a number of hours without being destroyed. Electronic chips and components are therefore frequently associated with a cooling circuit.
It has already been suggested to produce microchannels in at least part of one of the faces of the substrate of an electronic chip in a nonfunctional region of this chip. Microchannels are channels the largest transverse dimension of which is smaller than 500 μm and preferably smaller than 100 μm. Such microchannels are, for example, described in the following article:
Tuckerman D. B., Pease R. F. W., “High Performance Heat Sinking For VLSI”, IEEE Electron Devices Letters, volume EDL-2-5, pages 126-129, 1981.
In most cases, these microchannels extend parallel to the plane of the substrate of the chip so as not to interfere with the zones through which current flows. However, recently, in the field of integrated circuits, it has been suggested to pass these microchannels through the substrate, perpendicular to the plane of the latter (see for example patent application US 2009/0294954). In the latter case, the microchannels are carefully placed to avoid the one or more active zones of the integrated circuit, through which zones useful current flows. This is because the active zones of an integrated circuit are extremely small (about a few nanometers to a few tens of nanometers square). Thus, if a microchannel were to pass through or touch this active zone the latter would disappear. The latter technique therefore requires, at the design stage of the integrated circuit, provision to be made for nonfunctional, from the electronic point of view, areas through which microchannels can pass. These nonfunctional areas increase the footprint of the chip and make its miniaturization more difficult.
At the present time, to improve electronic chips, more effective cooling of these electronic chips is required. Moreover, microchannels lying parallel to the substrate are long, thereby leading to substantial head loss and reducing the heat exchange coefficient between the heat-transfer fluid and the substrate.
In addition, instead of using microchannels, electrical tracks that supply and collect the useful current that flows through the active zone are also used to evacuate heat from, and therefore cool, the active zone.
Because the electrical tracks have this dual function, contacts between these electrical tracks and the active zone must not only have a high electrical conductivity but must also be good conductors of heat. To ensure good conduction of heat, these electrical connections are produced by soldering or by adhesive bonding or at very high pressures. In addition, they generally have a large area in order to reduce their thermal resistance. Thus, such contacts are good thermal and electrical conductors and generally provide the chip with mechanical strength, and they are mechanically stressed by temperature variations. This is because the substrate and the electrical tracks generally do not have the same thermal expansion coefficients and there is no degree of freedom between them.
The prior art also includes patent applications EP 1 988 572 A1, US 2007/117306 A1 and US 2007/126103 A1.